FIG. 5 shows an example of a thermal printhead as related art. The illustrated thermal printhead B includes an insulating substrate 90 on which a glaze layer 91, a resistor layer 92, an electrode layer 93 and a protective layer 94 are successively formed.
The electrode layer 93 includes a plurality of electrode portions 93a and a plurality of electrode portions 93b which are spaced from each other to provide a gap therebetween where the electrode layer does not overlap the resistor layer 92. Of the resistor layer 92, the portion corresponding to the gap between the electrode portions 93a and the electrode portion 93b serves as a heat-producing resistance section 92a which is heated when energized. The heat-producing resistance section 92a is located on a bulging portion 91a of the glaze layer 91 against which a recording sheet S and an ink ribbon R are pressed by a platen roller P. This arrangement enhances the contact pressure between the recording sheet S, the ink ribbon R and the heat-producing resistance section 92a. 
The platen roller P is made of rubber, for example. Two strips of edge patterns 95 are provided downstream from the electrode portions 93b of the electrode layer 93 in a secondary scanning direction x which is the transfer direction of the recording sheet S and the ink ribbon R. The edge patterns 95 serve to prevent the glaze layer 91 from chipping at the edge thereof or the nearby portions during the manufacturing process of the thermal printhead B or the subsequent handling.
Patent Document 1: JP-A-H05-169698
The thermal printhead B has the following drawbacks.
As indicated by the arrows in FIG. 5, in performing printing on the recording sheet S, the recording sheet S and the ink ribbon R are transferred in the secondary scanning direction x while being pressed against the heat-producing resistance section 92a or the nearby portions by the platen roller P. The ink ribbon R has a relatively small thickness and is likely to be wrinkled. Therefore, when the ink ribbon R is transferred while being pressed against the thermal printhead B, wrinkles may be formed in the ink ribbon R.
Particularly, of the ink ribbon R, the portion heated by the heat-producing resistance section 92a expands and then shrinks due to the cooling by air. The shrinkage occurs also in the width direction of the ink ribbon R, which encourages the formation of wrinkles in the ink ribbon R.
Moreover, since the region where two edge patterns 95 are provided is bulged in the thermal printhead B, the ink ribbon R is pressed also against this portion strongly. Also for this reason, a force corresponding to the transferring force of the platen roller P is exerted on the ink ribbon R in the direction opposite from the secondary scanning direction x, which may result in the formation of wrinkles in the ink ribbon R. When the ink ribbon R is wrinkled and hence folded, ink cannot be properly transferred from the folded portion of the ink ribbon R to the recording sheet S, which results in print failure.